Well, I'm going to stick my torch in this fire, because I believe something different.
My understanding of the Creation story in the Bible is it is the explanation of the origin of God's chosen race of man and that Adam was the first of his chosen or favored people and Eden was a Paradise in which he placed Adam apart from the rest of the world which we know exists because when he and Eve transgressed he was sent away from the Garden into the world.
I believe God filled his Creation with living creatures including humans other than Adam and while Adam was created first and placed in Garden (set apart from everyone) he created others.
This is evident when Cain killed Abel and was sent to Nod to dwell among the people there. If this wasn't the case, where would those people have come from. Remember God said he "marked" Cain so they would know not to touch him.
As shodge said, the "Let us make man" comment does not refer to other men, but I believe it also does not necessarily refer to the Trinity which is a derived concept by the early Church.
I actually think God was speaking with other gods or beings of power whom God later forbade his chosen people from worshipping (The Exodus' Ten Commandments) Why make the commandment if God was the only one in the heavens? I think he (Father, Son and Holy Spirit) is all-powerful but he is not
Just my 2 cents